1. Field of the Invention
The present invention relates to a filter and a duplexer in which frequency-characteristics are variable, and a communication apparatus using at least one of them.
2. Description of the Related Art
A frequency-characteristics variable filter was suggested as disclosed in Japanese Unexamined Patent Application Publication No. 7-321509. In the suggested frequency-characteristics variable filter, reactance elements such as capacitors are connected to resonators via individual diodes, and voltages applied to the diodes are controlled, thereby varying resonant frequencies of resonators.
With a PIN diode used for each of the aforementioned diodes, the resonant frequency is switched between ON and OFF states, thereby providing two bands as frequency characteristics of the filter. Normally, when the PIN diode is in the ON state, a positive bias voltage is applied; and when the PIN diode is in the OFF state, a negative bias voltage is applied.
However, in a duplexer configured of the above-described frequency-characteristics variable filters, problems arise as described below.
With the above-described filter used for a transmitting filter, since a high power is inputted to the diode switch, the PIN diode switch having a high power-withstanding characteristic is used. When a high power is inputted to the transmitting filter, in addition to a serial bias voltage applied to a control terminal, a radio-frequency (RF) voltage is applied to each end of the diode switch. Therefore, generally, a high negative bias voltage of about xe2x88x9220 V must be applied thereto so that the diode switch can be maintained in a stable OFF state even when the diode switch receives a high transmitting-wave power in the OFF state.
However, a small and battery-driven apparatus, such as a portable telephone terminal, is required to be controlled using a negative power source with the lowest-possible voltage (with the smallest-possible absolute voltage value). A power in a range of 1 to 3 W is applied to a transmitting filter of the portable telephone terminal for transmission. However, when a low negative bias voltage of, for example, about xe2x88x923 V is applied to a diode switch in the transmitting filter, problems arise. The problems are that an interterminal electrostatic capacitance of the diode in the OFF state varies, frequency characteristics of the transmitting filter also varies, and in addition, the transmitting signal is skewed.
In a receiving filter of the portable telephone terminal, normally, a high power as in the case of the transmitting filter is not inputted. However, in a transmitting filter configured such that received signals are inputted from an antenna port commonly used for transmitting and receiving signals, a small amount of an RF voltage is applied from the transmitting side to the first resonator of the receiving filter. For this reason, when a low negative bias voltage is applied thereto, the frequency characteristics of the filter vary. In the unstable state where the interterminal electrostatic capacitance is large, two waves, that is, a transmitted wave and a disturbing wave from the antenna frequently cause intermodulation skews.
To overcome the above described problems, preferred embodiments of the present invention provide a filter and a duplexer in which the frequency-characteristics are variable, and a communication apparatus that can be operated at reduced bias voltages applied to diodes provided for switching frequency characteristics without reducing the stability of frequency characteristics and without increasing the occurrence of skews.
One preferred embodiment of the present invention provides a frequency-characteristics variable filter comprising: a plurality of resonators; a plurality of reactance elements respectively connected to the plurality of resonators via diodes; at least two types of the diodes having different characteristics corresponding to applied radio-frequency voltages.
According to the above, the diodes appropriate to the applied RF voltages are used. Therefore, frequency characteristics can be stably and surely switched without being influenced by characteristics of the diodes.
In the above described frequency-characteristics variable filter, the characteristics of each of the diodes may be characteristics of an interterminal electrostatic capacitance corresponding to the applied voltage. For example, for the diode to which a high radio-frequency (RF) voltage is applied, a diode having a small interterminal electrostatic capacitance is used. By this, even with a low bias voltage for turning off the diode, variation in the interterminal electrostatic capacitance of the diode can be reduced. This allows stable frequency characteristics and skew-reducing characteristics to be obtained.
Another preferred embodiment of the present invention provides a duplexer, wherein: the duplexer comprises two frequency-characteristics variable filters each comprising at least one resonators and at least one reactance elements connected to the at least one resonators via at least one diodes; one of the frequency-characteristics variable filters is a transmitting filter and the other one of the frequency-characteristics variable filters is a receiving filter; and at least one of the diodes in the transmitting filter has an interterminal electrostatic capacitance in the off state, the interterminal electrostatic capacitance being smaller than that of the diode connected to the resonators in the receiving filter excluding the resonator of a first stage.
According to the above, since the single duplexer can be used for two frequency bands, the overall miniaturization can be implemented. In addition, even with low bias voltages for turning off the diodes, reduction can be achieved for variation in the interterminal electrostatic capacitance when RF voltages as transmitting waves are applied to the transmitting filter. This allows stable frequency characteristics and skew-reducing characteristics to be obtained. Therefore, the duplexer can be suitably and easily used for small communication apparatuses, such as portable telephone terminals, which are driven by a low-voltage power source.
Yet another preferred embodiment of the present invention provides a duplexer, wherein: the duplexer comprises two frequency-characteristics variable filters each comprising at least one resonators and at least one reactance elements connected to the at least one resonators via at least one diodes; one of the frequency-characteristics variable filters is a transmitting filter and the other one of the frequency-characteristics variable filters is a receiving filter; and at least one of the diodes connected to the resonator of a first stage in the receiving filter has an interterminal electrostatic capacitance in the off state, the interterminal electrostatic capacitance being smaller than that of one of the diodes connected to the resonators in the receiving filter excluding the resonator of a first stage.
According to this, even with low bias voltages for turning off the diodes in the receiving filter, reduction can be achieved for variation in the interterminal electrostatic capacitance. Also, intermodulation skews occurring when a disturbing wave intrudes from the transmitting side into the first resonator of the receiving filter can be minimized.
In the above described duplexer, the interterminal electrostatic capacitance may be about 0.7 pF or less when an interterminal voltage of the diode is 0 V.
According to this, even with a voltage of the negative power source that applies reverse bias voltage to the diode, for example, with 0 V, frequency variations due to transmitting waves do not cause problems in the practical use. Therefore, the frequency characteristics of the filter can be switched using a negative power source that has a negative voltage represented by a small absolute value.
Yet another preferred embodiment of the present. invention provides a duplexer, wherein: the duplexer comprises two frequency-characteristics variable filters each comprising at least one resonators and at least one reactance elements connected to the at least one resonators via at least one diodes; one of the frequency-characteristics variable filters is a transmitting filter and the other one of the frequency-characteristics variable filters is a receiving filter; and one of the diodes connected to the resonators in the receiving filter excluding the resonator of a first stage has a forward resistance value smaller than that of each of the diodes connected to the resonators in the transmitting filter, and has an Q value in the off state higher than that each of the diodes connected to the resonators in the transmitting filter.
As described above, with the diodes provided at least for the resonators excluding the first resonator in the receiving filter, to which RF voltages as transmitting waves are not applied, no problems arise in variation in the interterminal electrostatic capacitance because of the application of the RF voltages. Therefore, loss can be reduced by using diodes each having the low forward resistance in the on state, and a high Q value in the off state of the interterminal electrostatic capacitance can be obtained.
In this way, losses at least due to the diodes provided for the resonators excluding the first resonator can be reduced. Also, since switching diodes for small signals can be used instead of PIN diodes, costs can be reduced.
Yet another preferred embodiment of the present invention provides a communication apparatus comprising either the above described filter or the above described duplexer. By this arrangement, communication with stable frequency characteristics can be performed in different frequency bands, and also, the small and low-loss type communication apparatus can be obtained.